Textures focal-conic

While such behavior can be seen in achiral LC phases, driven by surface constraints as illustrated in Figure 8.11, further characterization of the phase responsible for the minority domains in the B2 banana phases shows this to be the first unequivocal example of a bulk fluid conglomerate. It is worth noting here that the EO behavior of the majority domains is achiral Stripes parallel to the layers switch to a SmA-like focal conic texture, which is identical for both signs of the field, as can be seen in Figure 8.22. This EO behavior very clearly appears achiral when observing the switching in motion. 

Recently, Sato and Hatano 67 69) found a new type of chiral lyotropic mesophase composed of Tween 80, sorbitan mono-9-octadecenoate poly(oxy-l,2-ethanediyl), and water, and discussed the ICD of achiral solute molecules intercalated into the lyotropic mesophase. As the concentration of Tween 80 is increased, three distinct phases are obtained micelle, neat phase, and reversed micelle, in that order. In the region of the volume ratio of Tween 80/(Tween 80 + water) of 0.40 to 0.63 under crossed Nicol-prisms, a focal conic texture was observed. This result indicates that the... 

Fig. 9 Textures of the columnar phases of compound 34b. Pseudo-focal conic texture at 92 °C of the Colhd phase (bottom) and at 74 °C in the body-centred orthorhombic columnar phase (top). In order to increase the dynamic range of the photographs various photos with different exposure times have been superimposed. The black Maltese cross is perfectly dark in the centre for the Colhd- In the low temperature phase undulation of the columns produces birefringence in the dark branches...

Fig. 9-11. Representative thermal polarized optical micrograph of the focal-conic texture displayed by 14 (n = 12) in the smectic A phase on cooling from the isotropic liquid to 164 °C.

Ferrocene derivatives 15 exhibited remarkable liquid crystal properties (Fig. 9-13). Indeed, they all gave rise to enantiotropic mesophases. Structures with n = 1 to 11 showed nematic phases. From n = 12 a smectic C phase formed. The latter was monotropic only for 15 (n = 12). The smectic C domain increased from n = 13 to n = 16, and, inversely, the nematic range narrowed. The last member of this series (n = 18) presented one smectic C phase between 159 °C and 179 °C. A nematic to smectic C transition and a focal-conic texture of a smectic C phase are presented in Figs. 9-14 and 9-15, respectively. 

In the THF insoluble fraction (BP6Li), the observation of a focal conic texture as well as the presence of batonnets prior to clearing leads... 

The same authors increased the complexity of their systems by introducing in a polyester chain both ionic and chiral chain segments. The series containing both the isosorbide chiral units and the ionic moieties yielded chiral smectic C (SmC ) and chiral smectic B (SmB ) liquid-crystalline phases, exhibiting broken focal-conic texture and schlieren texture. Not surprisingly, the analogous polymer without the chiral units exhibited only the nonchiral SmC mesophase. On the other hand, in this case, the effect of ionic units on the phase behavior was negligible [91]. 

The cholesteric liquid crystal can be regarded as a layered structure so that it also shows the focal-conic texture, especially for large pitch cholesteric liquid crystals. 

Nevertheless in polymeric liquid crystals the same types of orientational defects and thus the same types of textures as present in the low mass counterparts have been observed. The textures often formed by polymers are the threaded texture, the schlieren texture and the focal conic texture of smectics. As is for low mass liquid crystals, the texture is a consequence of defects (disclinations and dislocations, refer to Chapter 1) present in the liquid crystal and is characteristic of a specific type of the phase. The texture examination has become a very useful tool in the determination of the type and nature of the polymeric liquid crystals. 

The formation of focal-conic textures is characteristic of smectic phases (Gray and Goodby, 1984). These textures are the result of smectic layers arranged in Dupin cyclides (Friedel, 1922). The common defects included in these arrangements have the form of ellipses and hyperbolae in certain confocal relationship. Dependent on the direction of observation relative to the defects, the dark lines shown in Figure 4.20 may be observed in focal-conic domains. 

Figure 4.21 shows a focal-conic fan texture of a smectic A phase in which is observable some defects represented by Figure 4.20(b). While Sa forms the fan-like focal-conic textures, the focal-conics formed by the Sc phase are often broken and less distinct. In addition, as stated above, Sa is uniaxial but Sc is biaxial. Sa may take a homeotropic alignment with the axis normal to the sample plane and extinct on POM with crossed polars. The two smectic phases are therefore distinguishable with POM. Nevertheless, it is desirable to include WAXS studies in order to identify the two with assurance (Section 4.3). 

Studies on low and high mass cholesteric phases have shown that the compound may form a focal-conic texture if no displacement is made on the cover slip when the sample is heated up or cooled down to the cholesteric phase. The focal-conic texture is similar to that of a smectic A, but the particles are usually too small to identify except for rare preparations. If the cover slip is displaced the focal-conic texture will give way to a homeotropic one with the optic axis normal to the plane of the preparation. The cholesteric homeotropic film reflects vivid colors having wavelength A decided by the De Vries (1951) expression A = pn where p is the pitch... 

The Sc phase exhibits four textures including the pseudo-homeotropic texture, the petal texture, the focal-conic texture and the lined texture (Gray and Goodby, 1984). The last occurs when the layer planes are not parallel to the surface. The lined appearance may be similar to what have been discussed for the cholesteric phase, but can also occur as being superimposed on the focal-conics. Examples of such lined and focal-conic... 

Giandjean texture (disordered) - focal conic texture (ordered)... 

Fig. 9-15. Representative thermal polarized optieal micrograph of the focal-conic texture displayed...

Plate 2. The contact regions between the (R)- and the (S)-l-methylheptyl 4 -(4-n-alkoxyphenylpropioloyloxy)biphenyl-4-carboxylates under a cover-slip on a microscope slide and between crossed polars (xlOO). The central region is where the two materials met and mixed. This region exhibits the normal focal-conic texture of the achiral smectic A phase. The left- and right-hand sides of the plate show the homeotropic and filamentary textures of the chiral TGBA phase... 

Fig. 1.1.7. Focal conic textures in smectic A. (a) The polygonal texture. Crossed polarizers. (Friedel. ) (6) Simple fan-shaped texture. Crossed polarizers. (Sack-...

Because of the layered structure, defects in the cholesteric can be likened in many respects to those in smectic A. Both of them exhibit focal conic textures and both allow for the existence of screw and edge dislocations. To discuss these similarities we employ a coarse-grained approximation in which the cholesteric distortions are considered to be small and to vary slowly over a pitch. In this approximation the free energy of distortion may be expressed in terms of layer displacement u parallel to the twist axis ... 

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